1. Field of the Invention
This invention relates to a process for the preparation of Nickel base superalloys and articles made thereof.
2. Description of the Related Art
The performance of gas turbine engines is limited by the high temperature capability of turbine blades and nozzle guide vanes (NGV) in the engines. A hot combustion gas-air mixture containing highly corrosive ingredients is directed at high pressure and velocity against the NGV parts in such engines. This hot gas then expands through the turbine blades and imparts its kinetic energy to the rotating turbine blades. Gas turbine blades and vanes therefore operate in extermely hostile environment of high temperatures, high stresses, oxidation and hot corrosion and accordingly, the materials development for such a critical application has been quite challenging. The directionally solidified (DS) columnar grained alloy CM 247 LC has been successfully used in the prior art Up to blade or vane metal temperature of about 1070.degree. C. Further improvement was possible after partial substitution of W and Cr from the CM 247 LC alloy composition by 3 wt. % of Re to result in the second generation columnar grained alloy CM 186 LC. About 16.degree. C. improvement over the temperature capability of CM 247 LC could be realised by the CM 186 LC alloy. This advantage is attributed to the presence of Re and a higher total refractory elements of 16 wt. % compared to that of 14.6 wt. % of CM 247 LC. Modern aero-engines with most advanced single crystal superalloys in their first stage turbine components however required far greater temperature capability than that of the CM 186 LC alloy for their subsequent stages of turbine parts which are often impractical to cast in single from.
Another disadvantage of the processes known in the prior art for preparing a hollow columnar grained component is the requirement of ceramic cores which leads to higher rejection of components.
Yet another disadvantage of the processes known in the art is the molten alloy and ceramic core reaction that leads to appreciable loss of key alloying elements which in turn deteriorates materials performance. Further disadvantage with the columnar grained materials of prior art is that a good fraction of Re and other refractory elements are not utilized for creep resistance as they are locked up in a coarse gamma prime phase, whose solutionization often leads to recrystallization and deleterious phase formation and deteriorates creep resistance.
The primary object of the present invention is to propose Ni-base superalloys and articles made therefrom with higher refractory element content beyond that of the CM 186 LC alloy in order to achieve superior creep resistance for applications such as gas turbine blades and vanes with adequate resistance to oxidation, hot-corrosion and deleterious phase formation.
Another object of the present invention is to propose Ni-base superalloys and articles made therefrom with excellent castability for advanced gas turbine blades and vanes in polycrystalline form having thin walled aero-foils, shrouded segments and intricate cooling channels.
A further object of the present invention is to propose a process for preparation of hollow columnar grained turbine components having complex cooling channels without employing ceramic cores and improving the yield of quality components.
Yet a further object of the present invention is to propose a process for preparation of hollow columnar grained materials which can be heat treated without causing any recrystallization to achieve improved balance of critical mechanical properties for advanced gas turbine engine application.
A still further object of the present invention is to propose columnar grained materials which can be conveniently brazed and given protective coating by existing manufacturing techniques during heat treatment.
Another object of this invention is to propose a columnar grained alloy composition out of the most advanced single crystal alloy chemistry and ensure thereby component castability as well as performance superiority over prior art materials.
These and other objects and advantages will be more clearly understood from the following detailed descriptions drawings and specific examples which are intended to be typical of, rather than in any way limiting on the scope of the present invention.